[System.Web] HeadersCollection read only exception

In reference source HttpHeaderCollection [1] when Add, Set or Remove is
called and the collection is readonly a PlatformNotSupportedException is
thrown.

Mono HeadersCollection was throwing NotSupportedException and
Microsoft.Owin.Host.SystemWeb.OwinCallContext.RemoveAcceptEncoding was
not catching the exception because it was expecting
PlatformNotSupportedException.

Fixes #33809

[1] http://referencesource.microsoft.com/#System.Web/HttpHeaderCollection.cs,73

mcs/class/System.Web/System.Web/HeadersCollection.cs

@@ -39,6 +39,30 @@ namespace System.Web
 		{
 		}
 
+		public override void Add (string name, string value)
+		{
+			if (IsReadOnly)
+				throw new PlatformNotSupportedException ();
+
+			base.Set (name, value);
+		}
+
+		public override void Set (string name, string value)
+		{
+			if (IsReadOnly)
+				throw new PlatformNotSupportedException ();
+
+			base.Set (name, value);
+		}
+
+		public override void Remove (string name)
+		{
+			if (IsReadOnly)
+				throw new PlatformNotSupportedException ();
+
+			base.Remove (name);
+		}
+
 		protected override void InsertInfo()
 		{
 			HttpWorkerRequest worker_request = _request.WorkerRequest;